The present invention relates in general to belt presses for dewatering slurries, and in particular to an improved drive and belt synchronizing system for such belt presses.
Belt presses generally consist of a frame having a slurry-receiving end and a slurry-discharge end and supporting several rolls or rollers of varying diameters mounted parallel to each other and transversely to the flow direction of the slurry through the frame. At least one and often two or more porous endless belts are supported by these rolls and, in multiple belt presses, each belt is associated with a specific set of rolls to perform a specific phase of the slurry dewatering process. In two-belt presses, an upper belt is normally positioned to travel a loop above the loop traveled by a lower belt. Most presses include a pressurized dewatering section, where two belts travel together with a layer of slurry sandwiched between them in a serpentine path over and under a generally horizontal series of wringing rolls of varying diameter. It is well established that belts are subject to increased tension and consequently increased dewatering pressure as the diameter of the wringing roll decreases.
In operation, a slurry having a relatively low solids content is deposited on the upper belt at the slurry-receiving end of the press, where means for draining free water are provided. The remaining solids are transported to the dewatering section, where the gradually increasing tension on the belts serves to wring most of the water from the slurry, leaving a dry cake which is scraped off the belt with a doctor blade.
Heretofore, it has been the usual procedure to power each belt by a drive roll positioned at the slurry discharge end in order to pull the belt through the dewatering section. U.S. Pat. Nos. 3,699,881 to Levin, et al. and 3,894,486 to Sparowitz, et al. describe two-belt presses in which the belts are driven in this manner.
In prior art presses using conventional drive means, the upper belt frequently moves slower than the lower belt due to a loss of tension from the heavier dewatering load under which it operates. Once a speed differential is established between the belts, there is a tendency for that differential to increase over time, thus lowering the dewatering efficiency of the press. It has also been observed that the upper and lower belts often slip against each other as they travel the serpentine path over and under the wringing rollers as a result of the above-mentioned load differential as well as the fact that each belt is alternately subjected to higher tension as it travels over one roll on top of the other belt, then slightly less tension as it passes directly against the next roll, with the other belt on top of it.
An often-encountered problem with prior art presses used to dewater slurries generated by coal mining is that as the slurry is dewatered, the concentrated solids form cakes of nonplastic material. This causes a phenomenon known as "lockup", wherein the dewatered material becomes lodged between the two belts, preventing them from sliding against each other. In a "lockup" situation, since the belts cannot slip against each other, they are forced to compensate for the press's inherent difference in belt tension by either stretching or bunching up. If a hard or sharp piece of slurry material becomes "locked-up", it will cut belt fibers as it forces them against the hard surface of the steel pressure rolls. Also, when the "lockup" is released, it has been observed that one of the belts will jump or jerk back into a more normal condition. This phenomenon is known as "popping". These resulting conditions increase belt wear and significantly decrease the efficiency of the dewatering section to the point where the last high pressure roller performs most of the dewatering. Moreover, this problem is aggravated by the above-mentioned relative speed differentials between the two belts.
In prior art presses using two drive rolls, one for each belt, to essentially pull the belts through the high pressure dewatering section of the press, the drive rolls are normally covered with rubber or a similar elastomer to enhance the driving friction between the belts and the rolls. In order to compensate for the "lockup" problem, and to prevent belt wear, the normally uncovered steel interior rolls are sometimes fitted with a plastic coating designed to alleviate the lockup problem by encouraging the belts to slip against the rolls, thus preventing belt stretch and wear.
A drawback of this system is that any slippage of the belts, either against the other belt or against a steel roll, will increase instead of decrease belt wear. An additional drawback of this method is that the plastic coating is not as compressible as rubber, and as it wears through it breaks up into sharp edges which cut the belt. Prior art presses were not able to cover the pressure rolls with elastomeric material because the friction caused by inherent belt slippage tended to wear away the elastomeric covering.
A further deficiency of prior art presses is their inability to compensate for solids having varying degrees of plasticity; for example, the more plastic the dewatered solid is, the more relative slippage is permissible between the belts.
Accordingly, a principal object of the present invention is to provide an improved belt press drive wherein the dewatering pressure increases along a more uniform gradient throughout the dewatering section.
A further object of the present invention is to provide a belt press drive wherein "lockups", the resulting popping and consequential belt wear and slippage are minimized.
A still further object of the present invention is to provide a belt speed monitoring and tension control system which regulates belt tension to keep both belts moving at generally the same speed through the dewatering section.